(1) Field of the Invention
The present invention relates to a system and method for automatically controlling vehicle speed to a set cruising speed.
(2) Background of the Art
Various types of systems and methods for automatically controlling vehicle speed to a set cruising speed have been proposed.
A Japanese Patent Application Unexamined Publication sho 59-8023 published on Jan. 17, 1984 exemplifies one of the proposed automatical vehicle speed controlling systems including a vehicle speed sensor outputting an operation data in proportion to the vehicle speed, a command switch group outputting cruise command signals, means for storing the operation data derived from the vehicle speed sensor in response to an operation of the command switch group, an actuator for actuating an engine throttle valve, and controlling means for providing a command to the actuator according to a difference between the actual vehicle speed and the stored vehicle speed.
In the automatical vehicle speed controlling system in the above-identified Japanese Patent Application Publication, the vehicle speed at a time when a set switch in the command switch is turned off after the set switch has been turned on is stored in a memory location, the controlling means provides the command for the actuator in accordance with a difference between the stored vehicle speed and actual vehicle speed, and the throttle valve is actuated in response to the operation of the actuator so that the actual vehicle speed is adjusted to the stored vehicle speed to achieve a cruise run of the vehicle at the stored vehicle speed. In addition, the actuator is forced in an acceleration state according to an off operation of the above-described acceleration switch and the vehicle runs at the cruising speed which is the vehicle speed at a time when the acceleration switch has been turned off.
However, the above-described previously proposed vehicle speed automatical controlling system has a drawback. That is to say, it is necessary to adjust an abrupt deceleration duration of time in which the actuator is rendered in a deceleration state immediately after the acceleration switch in the on state has, in turn, been turned off in a case when the actuator has been rendered in the constant acceleration state is response to the on operation of the acceleration switch for each vehicle in which the system is mounted since an engine performance, deceleration ratio, weight, and road condition are varied according to the vehicle in which the system is mounted. If such an adjustment is imperfect, the vehicle speed exhibits an undesired undershooting and/or overshooting. This is especially pronounced as the duration of abrupt deceleration increases. In this situation the vehicle speed exhibits increased undershooting of the desired speed and, consequently, is more difficult to drive.